The invention relates to a method for purifying a Redox mediator before electrolytic regeneration thereof during a chemical oxidation process of organic compounds.
It is well known that oxidation of unsaturated long chain fatty acids such as oleic acid, yields short chain fatty diacids, such as azelaic acid or pelargonic acid. Usually, such an oxidation is carried out with a solution of chromic (Cr VI) acid and sulfuric acid produced by electrolysis of an aqueous solution of chromium (Cr III) sulphate and sulfuric acid. During the electrolysis, hydrogen is released at the cathode and the chromium sulfate is converted into chromic acid and sulfuric acid at the anode (see U.S. Pat. No. 2,450,858 granted in 1948).
When this type of oxidation (also called indirect electrosynthesis) is used, it is customary to recover the solution of chromium sulfate and sulfuric acid obtained after the oxidation reaction. The so-recovered solution is then recycled towards an electrolysis cell in order to regenerate it to obtain the desired ionic species for the oxidation reaction.
A major difficulty of this process of recovering and regeneration of the electrolyte lies in the presence of organic impurities in the electrolytic solution. It turns out that these impurities accumulate and deposit on the electrodes of the electrolysis cell. This results in a reduction of the electrolysis current efficiency.
It is also known that solutions of Cerium III/Cerium IV are used for the electrochemical oxidation of other types of organic products. Electrochemical oxidation of other types of organic products such as, but not limited to aromatic aldehydes and quinones as described by HARRISON in U.S. Pat. No. 5,296,107 and by KREH et al in U.S. Pat. Nos.: 4,639,298; 4,647,349; 4,620,108; 4,701,245 and 4,794,172. The commercial applicability has been described by HARRISON in Journal of new Materials for Electrochemical Systems, January 1999.
Work done by the LTEE in collaboration with W. R. Grace and Co. has led to the development of a generic technology for the selective manufacture of a whole array of chemical products of high interest. The success of this technology has already been confirmed during pilot projects directed to the synthesis of anthraquinone, aminoanthraquinone and para-tolualdehyde.
However, even in these cases, the presence of soluble organic compounds in the mediator solution recovered at the end of the reaction considerably affects the regeneration efficiency of the catalyst in an electrolytic cell. More precisely, these compounds affect the current efficiency and the lifetime of the electrodes and they generate drawbacks of operation in continuous mode. It turns out that the presence of organic compounds can block the surfaces of the electrodes and therefore reduce the production rate of the Redox mediator. To maintain this rate, the current must be increased.
In light of the above, it is therefore obvious that it is essential to purify the electrolytic solution recovered after the oxidation reaction before regenerating it.
In order to do it, it has already been suggested to treat the mediator solution with activated charcoal to absorb the dissolved organic molecules present in it. Although this method seems to work, it has a number of drawbacks.
First of all, it is costly because the consumption of activated charcoal expressed in kg per kg of obtained product is high (this is a direct consequence of the low electrolyte concentration and therefore of the high Velectrolyte/kgproduct ratio).
Secondly, after use, the activated charcoal must either be discarded or regenerated, but before doing so, the redox mediator must be recovered for economic and environmental reasons. Such can be done by washing the activated charcoal with water. However, concentrating the recovered redox reagent by evaporation is energy intensive. Handling of wet activated charcoal is also a labour intensive activity (this is a time consuming operation that results in higher labor costs and production costs).
Thirdly, this method cannot be used on non organic molecules and non-aromatic products having a low affinity for activated charcoal.
The object of the invention is to solve the above-mentioned problem that occurs in all the chemical processes that use electrochemically regenerated Redox couples.
More precisely, the object of the present invention is to provide a method that is both simple and efficient to solve the problem that was previously mentioned. This method essentially consists of purifying the solution containing the Redox mediator that is recovered from the oxidation reactor before introducing it into the electrolysis cell where the regeneration takes place. This purifying step is of a great importance since it allows removal of organic impurities that tend to deposit on the electrodes or consume regenerated mediator if they are not extracted from the electrolytic solution and which can therefore affect the operation of the electrolysis cell and the efficiency of the regeneration.
Thus, the method according to the invention is devised to purify a Redox mediator used in a chemical oxidation process of organic compounds before this mediator is regenerated electrochemically in an electrolysis cell. This method comprises the steps of:
recovering the mediator in the form of a solution containing volatile, soluble and insoluble impurities;
subjecting the solution containing the mediator to a thermal treatment, this thermal treatment being carried out in a reactor kept at a temperature high enough to allow oxidation of the impurities by means of left-over unused mediator that is still present in the solution under its oxidative form, or by addition of a given amount of said regenerated mediator and simultaneously to allow elimination of the volatile impurities present in the solution; and
filtering the solution that contains the mediator, to remove the insoluble impurities therefrom.
This series of steps constitutes the heart of the invention since it considerably increases the technical and economical feasability of the whole chemical process.
In this connection, it is worth mentioning that this method for purifying an electrolyte, is generic, economical and compatible with all the processes used for the treatment of an electrolyte in view of efficiently recycling it.